The present invention is directed to the integration of a pause detector in an antenna interface of a contactless device.
The basic components of a contactless card system are a contactless reader and the contactless card. The contactless reader, also known as a PCD, includes an antenna electrically coupled to an electronic circuit. The contactless card, also known as a smart card, a tag, a PICC, or an RFID tag, has an inductive antenna and an integrated circuit electrically coupled to the inductive antenna.
When the contactless card penetrates a transmission field of the reader, the reader antenna transmits to the contactless card a carrier signal, which generates a radio frequency (RF) field to supply the contactless card with power and data, which is achieved by amplitude modulation of the carrier signal. In return, the contactless card transmits data by load modulating the carrier signal. This load modulated signal is detected by the reader antenna. The communication between the reader and the contactless card may be defined by any of numerous ISO (International Organization for Standardization) standards, such as 14443 Type A/B/C, 15693, 18000, etc.
The ISO 14443 Type A communication protocol uses amplitude-shift keying (ASK) with a reader to contactless card modulation index of up to 100%. A single bit of data is coded as a pause in the transmission. During the pause, the carrier field emitted by the reader antenna is reduced by the modulation index. At a modulation index of 100%, the carrier field is therefore turned off during a pause, which can last several microseconds.
A conventional contactless card detects a field pause using a demodulator located separately from the contactless card antenna interface. The modulated carrier signal received by the contactless card is rectified and low-pass-filtered. The low-pass-filtered signal is then provided to a comparator circuit which generates a signal indicating the presence of a field pause.